1. Field of the Invention
The present invention generally relates to a vehicle steering apparatus for turning (steering) a wheel (normally the front wheels) of a vehicle with a turning unit that moves in response to a steering input applied to a steering wheel or other steering operating member by a driver. More specifically, the present invention relates to a variable gear ratio steering apparatus having a torque transfer ratio, i.e., a steering gear ratio, between the steering operating member and a rotary portion of the turning unit.
2. Background Information
One example of a variable gear ratio steering apparatus for a vehicle is disclosed in Japanese Laid-Open Patent Publication No. 2005-162124. In this publication, the variable gear ratio steering apparatus disclosed as having a gear ratio varying unit arranged between a steering operating member and a wheel turning unit such that the steering operating member and the wheel turning unit are coupled together through the gear ratio varying unit. The gear ratio varying unit serves to establish a torque transfer ratio between the rotation of the steering operating member side (input side) of the gear ratio varying unit and the rotation of the turning unit side (output side) of the gear ratio varying unit. The gear ratio varying unit is configured such that the torque transfer ratio, i.e., the steering gear ratio, can be changed in accordance with a traveling state (condition) of the vehicle. The gear ratio varying unit typically uses a reduction function to change the transfer ratio between an input rotation and an output rotation and, thus, a differential gear mechanism is the main component element. In the variable gear ratio steering apparatus mentioned above, a harmonic drive gear mechanism is used as the differential gear mechanism.
The vehicle steering apparatus is also typically provided with a mechanism for supplementing a steering torque that is transferred from the steering operating member to a wheel through the wheel turning unit such that the driver can experience a light and comfortable steering operation. Steering assistance devices for supplementing the steering torque have generally been hydraulic devices, but electric power steering systems employing an electric motor are becoming more and more popular.
The variable gear ratio steering apparatus presented in Japanese Laid-Open Patent Publication No. 2005-162124 employs an electric power steering system. The steering torque assistance motor of the variable gear ratio steering apparatus presented in Japanese Laid-Open Patent Publication No. 2005-162124 is arranged on the wheel turning unit, which is a rack and pinion type wheel turning unit. The steering torque assistance motor is arranged to apply a supplemental assistance torque to a steering rack of the wheel turning unit.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved variable gear ratio steering apparatus. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.